Martin Kyte | 24/02/2017 15:09:37 |
![]() 3445 forum posts 62 photos | mmm. but the propagation velocity depends on the dielectric constant so it's slower for 2.5mm twin and earth. Martin |
Russell Eberhardt | 24/02/2017 16:38:51 |
![]() 2785 forum posts 87 photos | Posted by Martin Kyte on 24/02/2017 15:09:37:
mmm. but the propagation velocity depends on the dielectric constant so it's slower for 2.5mm twin and earth. Martin Yes, the propagation velocity will be about 60% of that in free space so the wavelength at 50 Hz will be about 10,000 km. Russell |
Brian Oldford | 24/02/2017 17:42:30 |
![]() 686 forum posts 18 photos | Posted by Russell Eberhardt on 24/02/2017 16:38:51:
Yes, the propagation velocity will be about 60% of that in free space so the wavelength at 50 Hz will be about 10,000 km. Russell I'm afraid that is incorrect. The cable length will be shorter by the velocity factor i.e.approximately 3600km. Regards Brian (Retired Telecommunications Professional) Edited By Brian Oldford on 24/02/2017 17:45:53 |
Geoff Theasby | 24/02/2017 18:00:35 |
615 forum posts 21 photos | Haha! Why am I thinking of Cerenkov radiation? Anyway, coaxial stubs, filters, Hi-Q breaks, on 144 & 432 MHz Geoff |
Neil Wyatt | 24/02/2017 18:13:31 |
![]() 19226 forum posts 749 photos 86 articles | v=fλ So decreasing velocity decreases wavelength. Neil
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Brian Oldford | 24/02/2017 18:21:06 |
![]() 686 forum posts 18 photos | Posted by Geoff Theasby on 24/02/2017 18:00:35:
Haha! Why am I thinking of Cerenkov radiation? Anyway, coaxial stubs, filters, Hi-Q breaks, on 144 & 432 MHz Geoff Or even an odd integer 1/4 wave length of 75 ohm phasing harnesses for a pair of stacked yagis.
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Bob Brown 1 | 24/02/2017 18:39:51 |
![]() 1022 forum posts 127 photos | High current interconnectors use DC to transmit power over long distance then covert it back to AC to reduce losses.
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Geoff Theasby | 24/02/2017 22:08:18 |
615 forum posts 21 photos | No, Neil, in a given medium, 'free space', open wire line, coaxial cable etc., the velocity is the same for all frequencies. In a comparison between mediums, the velocity may be different, one to another, and a resonant or tuned length will be longer or shorter by that factor. Geoff
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Martin Whittle | 24/02/2017 23:04:33 |
102 forum posts 12 photos | The speed of an electromamgnetic wave (so radio, light, x rays, gamma rays etc) in vacuum is approximately 300,000km/sec. In a cable the speed is then dependant on the velocity factor, which is a ratio of the speed of the wave in the cable related to that in vaccuum. In most typical cables or other electromagnetic media (fibreoptic cable, lenses, etc) the speed is proportional to the square root of the dielectric constant of the material (also the permeability for case of magnetic materials). So most cables have insulation of plastic (polythene, PTFE, etc) having dielectric constant around 2 to 2.5, so the velocity factor is around typically 70% for solid dielectric, or maybe 75% for air-spaced plastic dielectric. The velocity factor also simply the inverse of the refractive index of the material: so for a dielectric constant of say 2.25, the refractive index is 1.5, and the velocity factor is 0.667. (Assuming the material is non-magnetic). So since the speed of light is 300km/sec or 186,000 mile/sec: if you connect yourself to a mile of cable and then have high voltage power applied at the other end, it will take 5.3 microseconds before you are electrocuted for open wires, or 7 microseconds through typical plastic-insulated cable. If the the cable were 70 light-years long, you then need not worry Martin Edited By Martin Whittle on 24/02/2017 23:06:42 Edited By Martin Whittle on 24/02/2017 23:07:34 Edited By Martin Whittle on 24/02/2017 23:20:06 |
Russell Eberhardt | 25/02/2017 11:58:12 |
![]() 2785 forum posts 87 photos | Posted by Russell Eberhardt on 24/02/2017 16:38:51:
Yes, the propagation velocity will be about 60% of that in free space so the wavelength at 50 Hz will be about 10,000 km. Embarrasing error for a retired electronic engineer. Must have had too much wine with my lunch Russell |
Gordon W | 25/02/2017 12:03:42 |
2011 forum posts | Will I have to check the speed of light before I can use my extension lead? It's near 50 metres long with rubber outer. |
Hacksaw | 25/02/2017 12:12:15 |
474 forum posts 202 photos | yes ,and the back e.m.f ionisation differential. |
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